This is a veterinarian that has clinical experience in diagnosing and treating illnesses in the various exotic mammal species (hamsters, ferrets, rabbits, rats, mice, etc.)
There are exotics exclusive practices, but that is the exception and not the norm. With that being said, some veterinarians that primarily see small animals (cats & dogs)
also have a special interest in exotic mammals. Having access to a veterinarian with this expertise reduces the likelihood of getting no diagnosis, a misdiagnosis, or potentially
harmful (and even fatal) medicine for your exotic mammal.
There is a path that a veterinarian can pursue to achieve certification as an Exotic Companion Mammal diplomate. The American Veterinary Medical Association granted it provisional
recognition in 2008 and to date there are less than 50 ECM diplomates worldwide.
For a veterinarian's view on what makes a vet an "exotics" veterinarian, we recommend that you read this.